The avalanche photodiode (APD) is a semiconductor device that can detect extremely low levels of electromagnetic radiation. Unlike a PIN photodiode, which generally produces a single electron for each photon received, an APD is constructed so that an electron dislodged by a photon will hit other atoms in the silicon of the APD with sufficient velocity and energy so that additional hole-electron pairs are created by the collisions. Typically a free electron will create a number of hole-electron pairs, and the electrons from these pairs will, in turn, create additional electrons, thus creating an "avalanche" process. This multiplication of electrons gives the APD an effective gain and allows detection of very low light levels.
Recent advances in the fabrication and performance of avalanche photodiodes (APD's) have led to their use in the detection of individual photons and other short-duration events. Although APD's have obvious advantages over photomultiplier tubes, including power requirements, size, and quantum efficiency, until recently, photomultiplier tubes have been required to get sufficient sensitivity and speed in these applications. When used in photon detection applications, APD's are frequently used in "Geiger" mode in which the APD is reverse biased with a voltage that exceeds its breakdown voltage. In Geiger mode, some means is necessary to stop or "quench" the current flowing through the diode after each avalanche is triggered by the received EM radiation.
One method to quench the current is to limit the maximum current flowing through the diode, by means of a series resistor, to a low enough level that the current will spontaneously cease due to the statistical nature of the avalanche process. While using simple circuitry, the minimum interval between detectable events is limited by the so-called "dead time," which is the time required to turn off the diode completely and to recharge the diode and parasitic capacitances through the typically large resistance of the current limiting resistor. To this end, various active circuits for quenching the APD current have been developed. Prior art active quenching circuits have been a compromise between the bias above breakdown, the "overvoltage," and the dead time, and are generally more complex than the present invention. The present invention can detect events separated by a few tens of nanoseconds with APD overvoltages on the order of tens of volts.